ath79: add correct phy-mode for QCA9563 boards

The TP-Link RE450 as well as the UniFi AC series missed the phy-mode
property. Because of this, the incorrect MII phy-mode from the root dtsi
was used.

With Kernel 5.4, this leads to problems when used with a AR8033 PHY. The
bootloader seems to leave the fiber pages selected.

As there's not switch to copper pages happening in at803x_config_init
due to the incorrect phy-mode, the new at803x_read_status will interpret
the status of the SGMII side as the status of the copper side.

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/ath79/dts/qca9563_tplink_re450-v2.dts

@@ -180,6 +180,7 @@
 &eth0 {
 	status = "okay";
 
+	phy-mode = "sgmii";
 	phy-handle = <&phy4>;
 
 	mtd-mac-address = <&info 0x8>;

target/linux/ath79/dts/qca9563_ubnt_unifiac-lite.dtsi

@@ -22,5 +22,6 @@
 	status = "okay";
 
 	mtd-mac-address = <&art 0x0>;
+	phy-mode = "sgmii";
 	phy-handle = <&phy4>;
 };